Ultrasonic inspection of rough surface aluminium die castings

Abstract

Most ultrasonic techniques developed in the die casting industry were to inspect castings with simple geometries and machined surfaces. A technique could not be described as non-destructive if it required alteration of a surface prior to inspection. This paper describes an investigation into the possibility of using an ultrasonic sensing-based non- destructive testing system to detect defects in aluminium die casting parts with rough surfaces. As the signal-to-noise ratio is very important in quantifying sub-surface defects, the major factors relating to noise creation in the ultrasonic signal during ultrasonic immersion testing were identified and addressed. The application of ultrasonic testing to inspect rough surface aluminium die castings with side- drilled holes to simulate porosity type defects is described in detail. Investigations indicate that the scattering from the rough surface of the castings masks the defect signal when the defect was close to the front surface. The results obtained from ultrasonic immersion testing indicated that focused ultrasonic probes operating at frequencies between 5 MHz and 10 MHz were best suited for the inspection of castings with surface roughness R a values varying between 50 µm and 100 µm.